Delivery of potentially therapeutic proteins to the brain is hampered by the blood-brain barrier (BBB). For example, the 16 kDa protein leptin acts within the CMSto control adiposity. However, obesity is associated with a peripheral resistance to leptin in which subjects are responsive to leptin injected into the brain but no longer respond to leptin given peripherally. We (Banks) and others have shown that peripheral resistance is explained by impairment in the endogenous BBB transporter for leptin. We have used a model protein, horseradish peroxidase (HRP) to show that coupling to a small number of the fatty acid residues (stearate) or Pluronic block copolymer chains improves the ability of proteins to cross the BBB. Unmodified HRP does not cross the BBB. We propose here to use HRP as a model to optimize the modifications of polypeptides with 1) fatty acids and 2) Pluronic that increase polypeptide permeability across the in vitro cell monolayer model of the BBB (SA 1) and to determine the mechanism(s) by which such modifications enhance BBB permeability (SA2). We will then determine in vivo the extent to which the most promising modifications improve both BBB transport rate and the accumulation by brain of both HRP and leptin (SA3). Finally, in SA4, we will determine the extent to which modified leptins retain biological activity after both peripheral administration and direct injection into brain. These studies will determine the usefulness of fatty acylation and conjugation with Pluronic for protein delivery to the CMS as well as the mechanisms responsible for increased BBB permeability. The prospective in the development of a specific drug (leptin) for treatment in a specific disease (obesity) will also be determined.